Drug-resistant Plasmodium falciparum malaria exerts a devastating impact in intertropical regions. Resistance to chloroquine (CQ) has led to a current emphasis on artemisinin-based combination therapies (ACTs), which combine a highly potent but short-lived artemisinin derivative with a less potent but longer-lasting partner drug. Remarkably, ACT regimens have been selected empirically and the contribution of PK/PD properties to ACT treatment outcomes has been largely understudied. Here, we propose a systematic analysis of PK/PD properties of antimalarial combinations and their relationship to treatment outcome and parasite resistance. In Aim 1, we will study ACTs. In vitro studies with P. falciparum will define potencies, rates of action and drug-drug interactions with ACT agents. We will assess in vivo PK/PD properties in a P. berghei BALB/c mouse model to identify optimal combinations and dosing regimens of existing ACTs, and search for improved combinations. ACTs will be evaluated in models of primary infections and re-infections with a drug-sensitive strain. PK/PD relationships will include comparisons of parasite response to drug (parasite clearance times, rates of recrudescence, survival) and drug exposure profiles (drug plasma levels including Cmax, concentration-time functions (AUC), time above minimum inhibitory concentrations). In Aim 2, PK/PD relationships will be determined for existing ACTs, as well as novel combinations identified in Aim 1, in primary infection and re-infection models against drug-resistant lines of P. berghei. These include lines that have acquired stable resistance to partner drugs or that are transiently resistant to artemisinins. We will also exploit a model of P. falciparum propagation in NODscid/[unreadable]2m-/- mice to assess in vivo efficacy against artemisinin and CQ-resistant P. falciparum. Resistance and toxicity concerns with ACTs make it necessary to consider alternative antimalarial combination therapies. Based on recent findings from Malawi, we propose that CQ may again be useful, if combined agents effective against CQ-resistant parasites. In Aim 3, we propose PK/PD studies to evaluate several candidate CQ-containing combinations. This will employ a CQ-resistant line of P. berghei in BALB/c mice, as well as the P. falciparum NODscid/[unreadable]2m-/- mouse model, to define whether these combinations can successfully treat multidrug resistant parasites, and define PK/PD correlates of treatment outcome. This aim will also use the BALB/c model to define the contribution of immunity. These studies should substantially enhance our understanding of the intricate relationship between antimalarial pharmacology, treatment outcome and the emergence of resistance, and provide robust data to guide the selection and optimization of antimalarial combinations to effectively treat and suppress drug- resistant malaria. This R01 grant application proposes to study the pharmacological properties of antimalarial drugs in clinical use. These properties will be assessed with human and rodent malaria parasites and will use mouse malaria models to identify pharmacological/ pharmacodynamic correlates of treatment outcome. These data will identify drug combinations and treatment regimens that reduce the emergence of drug resistant malaria, which poses a substantial burden on public health in tropical countries.